The present invention generally relates to database servers and, more particularly, to a hybrid architecture for video on demand (VOD) server which combines the best features of massively parallel processing (MPP) and workstation designs into a cost effective high performance system.
One potential service to be provided by the emerging national information infrastructure (or the so-called "information superhighway") is video on demand (VOD). VOD (or near-video on demand (NVOD)) is a service in which a viewer selects from a large catalogue of available movies, and the selected movie is almost immediately ready to be displayed on the viewer's home television set. The viewer interactively controls the playing of the movie in the same manner supported by video cassette recorders (VCRs). Thus, the VOD service acts as a virtual VCR coupled to a virtual video rental store. A viewer can watch any movie at any time with a few keystrokes on a home terminal device. A survey of VOD systems is presented by Thomas D.C. Little and Dinesh Venkatesh in "Prospects for Interactive Video-on-Demand", IEEE Multimedia, Fall 1994, pp. 14-24.
FIG. 1 shows a functional block diagram of a VOD system. At the heart of the system is the video server 10 which routes the digital movies, resident in the movie storage system 11, to the distribution infrastructure 12. This distribution infrastructure may be, for example, a telephone network and/or a cable TV (CATV) system, preferably employing optical fiber and implementing asynchronous transfer mode (ATM) transmission protocols. The distribution infrastructure 12 delivers movies to individual homes based on the routing information supplied by the video server 10. There is a set top box 13 resident in each subscriber's home. This box receives and decodes the digital movie and converts it to a signal for display on a TV set or monitor 14. In addition, the system includes a "back channel" through which a viewer orders and controls the playing of the digital movies. The back channel routes commands from the set top box 13 back to the video server 10 via the distribution network 12. An alternative backchannel could be implemented with existing communications facilities such as telephone lines.
The primary function of the video server 10 is to route compressed digital video streams from their storage location to the requesting viewers. In addition to this basic function, the video server 10 should support the virtual VCR functions of the VOD system. At a minimum, the VCR functions to be supported are start, pause/resume, fast forward, and rewind.
There are two primary design approaches, currently under development, for the video server; these are the massively parallel processor (MPP) approach and the workstation approach. The primary practitioner of the MPP approach is Oracle Corporation running relational database management software on an nCUBE MPP. The Oracle system is being tested in several trial VOD systems, including the Bell Atlantic trial in the Washington, D.C., area. The distinguishing characteristics of the Oracle/NCUBE approach are the following:
The MPP consists of a few thousand independent computing elements, each with its own central processing unit (CPU) and memory. Each computing element hosts a database program that independently reads movies from a disk and then routes the movies to a destination line. PA1 Each movie is partitioned (striped) across several disks so as to reduce contention for disk access among the computing elements. While the MPP approach seems to satisfy the requirements for the video server, the cost is high, requiring as it does a parallel relational database program in order to accomplish VOD data distribution. PA1 Each switch input and output must be able to sustain the 5.6 MB/sec data rate required to support multiple simultaneous compressed video streams. PA1 The switch must be able to map any permutation of the N inputs to the M outputs in a non-blocking fashion. PA1 The switch must be dynamically reconfigurable in real time, or near real time, upon command of the control workstation in order to support disk striping and a varying set of VOD subscribers.
Several trial VOD systems are exploring workstation based approaches. The Time Warner trial in Orlando, Fla., uses a Silicon Graphics Incorporated (SGI) workstation based server. A NYNEX trial will be based on a Digital Equipment Corporation (DEC) Alpha workstation, and a Pacific Telesys trial will use a Hewlett-Packard (HP) workstation. The Time Warner video server is illustrated in FIG. 2 and consists of five SGI workstation servers 20.sub.1 to 20.sub.5. Each workstation has a dedicated ATM connection to an AT&T ATM switch 21. The ATM switch 21 drives lines to the distribution infrastructure 22. Each workstation acts as an interface with the address of a specific set top box. The ATM switch 21 then routes the packets to the specific line that serves the neighborhood access node (NAN) 23.sub.1 to 23.sub.n for the particular set top box. To the extent understood by the inventors, Modulators in FIG. 2 are used to modulate the video signals sent to the set top boxes over the cable network. Demodulators demodulate user input signals sent from set top boxes over the cable network.
Again, the workstation based server appears to meet the video server requirements, although the workstation throughput appears to be exhausted. From a cost standpoint, this approach is expensive requiring, as it does, high performance CPUs resident in the workstations which are only lightly utilized. Most of the workstation time appears to be used in direct memory accessing data from the disks to the ATM adapter. High performance servers are used so that the workstation backplane can support the data throughput requirements. Additionally, the ATM switch provides much higher data rates than are needed for the VOD application.